Can’t find old Apple Watch and want to unpair to pair with new watch

Tap the Watch app on your iPhone. Tap All Watches at the top of the My Watch screen. Tap the info button to the right of your old watch. Select Unpair Apple Watch. Press Unpair to confirm the process.

To unpair and erase watch and remove Activation Lock

From a web browser,sign in to iCloud.com with your Apple ID.

Go to Find iPhone.

Click All Devices at the top of the screen.

Click your Apple Watch.

Click Erase [device]. Select Next until the device is erased.

Click Remove from Account.